This invention relates to electrical filters which have band selective transfer characteristics and which can be adjusted to alter the stop-band or pass-band. The invention particularly though not exclusively concerns the tuning or narrow-band rejection filters such as notch filters.
One example of the use to which a particular form of the invention may be put is the tuning of a radio navigational receiver to provide the greatest possible attenuation of signals at a particular, adjustable, frequency. Notch filters, that is to say narrow-band rejection filters, are often used to eliminate, or at least severely attenuate, interference which occurs at a particular frequency or within a narrow band of frequencies. Often the receiver's operator knows in advance the frequency of likely interference; nevertheless, it has proved difficult to provide a satisfactory means by which the notch filter can be adjusted to provide maximum rejection of signals at the interference frequency. One object of the present invention is to reduce that difficulty. The invention may have a wider utility, in particular in the tuning of a narrow-band pass filter.